P7: Functionalization, Doping, and Nanoribbons
Chair: Mauricio Terrones
- Wednesday PM, April 3, 2013
- Moscone West, Level 2, Room 2010-2012
1:45 PM - *P7.01
Atomically Precise Graphene Nanoribbons
Graphene nanoribbons (GNRs) - narrow stripes of graphene  - are predicted to be semiconductors with an electronic band gap that sensitively depends on the ribbon width . For armchair GNRs (AGNRs) the band gap is inversely proportional to the ribbon width. Furthermore, a quantum confinement-related periodic modulation of the band gap is added to the inverse scaling. This modulation has a period of three carbon dimers (ΔN=3) in ribbon width and becomes dominant for AGNRs narrower than ~3 nm . This allows, in principle, for the design of GNR-based structures with specific and widely tunable electronic properties, but requires structuring with atomic precision. The inability to produce graphene nanostructures with the needed precision has so far hampered the experimental investigation of the electronic structure of narrow GNRs, which is in contrast to an impressive number of GNR-related computational studies.
In this presentation, I will report on our recently developed bottom-up approach to the fabrication of atomically precise GNRs . It is based on a surface-assisted synthetic route using specifically designed precursor monomers, and has made available ultra-narrow GNRs and related graphene nanostructures for experimental investigations of their structural and electronic properties [4-10]. Using experimental methods such as scanning tunneling microscopy and spectroscopy, Raman and photoelectron spectroscopies, which are complemented by density functional theory calculations, we have studied the on-surface chemical reaction steps and the resulting graphene-related materials [4-10]. For the case of N=7 AGNRs, the electronic band gap and dispersion of the occupied electronic bands have been determined with high precision . Finally, it will be shown that the surface-chemical route also gives access to substitutionally doped ribbons as well as heterostructures.
 A. K. Geim, Science, 324 (2009) 1530.
 V. Barone et al., Nano Lett., 6 (2006) 2748.
 L. Yang et al., Phys. Rev. Lett., 99, (2007) 186801.
 J. Cai et al., Nature, 466 (2010) 470.
 M. Bieri et al., Chem. Commun., (2009) 6919.
 M. Bieri et al., J. Am. Chem. Soc., 132 (2010) 16669.
 S. Blankenburg et al., Small, 6 (2010) 2266.
 M. Treier et al., Nature Chemistry, 3 (2011) 61.
 S. Blankenburg et al., ACS Nano, 6 (2012) 2020.
 P. Ruffieux et al., ACS Nano, 6 (2012) 6930.
2:15 PM - P7.02
Bottom Up Synthesis of Nitrogen Doped Graphene Nanoribbons
In recent years graphene has attracted tremendous interest within the scientific community because of its exceptional electronic and optic properties. For example, high charge carrier mobility in connection with a high transparency makes it suitable for window electrodes. Moreover, doping of graphene with heteroatoms has further led to new materials suitable for catalysis and energy storage applications. Nitrogen doped graphene as a metal free catalyst material has proven to be a promising candidate for the oxygen reduction reaction (ORR) at the cathode in fuel cells.
In order to understand the catalytic properties in regard to the molecular structure of the catalytic centers we are currently working on a bottom-up synthesis of different nitrogen containing nanographenes and graphene nanoribbons. For this, we apply techniques of solution or surface assisted polymerization to gain polyphenylene polymers which upon ring-closure yield the desired graphene materials. In contrast to other protocols for example the treatment of graphene or graphene oxide with nitrogen sources like ammonia or the pyrolysis of nitrogen enriched precursors, the heteroatoms are at designated positions in graphene. In our approach the periphery thus can be designed by the synthesis of the monomers to be exclusive pyrimidine or pyridine type as one example.
It shall further be highlighted that nanographenes with nitrogen in a zig-zag periphery are accessible in our approach. This structure is a formal azomethine ylide and has been embedded in a polyaromatic framework for the first time. As it exhibits a dipolare structure its addition reaction towards oxygen is currently investigated. Oxygen tends to add to nitrogen doped graphene in a side-on fashion. These new nitrogen doped nanographenes shall help to model the reaction in order to understand the basic molecular mechanism of the oxygen reduction reaction.
In summary, we can extend our established method of bottom-up synthesis to a new class of nitrogen containing nanographenes and graphene nanoribbons.  
 M. G. Schwab, A. Narita, Y. Hernandez, T. Balandina, K. S. Mali, S. De Feyter, X. Feng, K. Müllen, Journal of the American Chemical Society 2012.
 Z.-S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng, K. Müllen, Journal of the American Chemical Society 2012, 134, 9082-9085.
 J. M. Cai, P. Ruffieux, R. Jaafar, M. Bieri, T. Braun, S. Blankenburg, M. Muoth, A. P. Seitsonen, M. Saleh, X. L. Feng, K. Müllen, R. Fasel, Nature 2010, 466, 470-473.
2:30 PM - P7.04
Inherent Schottky and Diodic Transistor Behavior in Tapered Graphene Nanoribbons: Fabrication and Characterization
Graphene nanoribbons (GNRs) are strips of single atom thick sp2-hybridized carbon atoms with band-gap governed by their width. We demonstrate that GNRs with tapered morphology exhibit semiconducting-to-metallic continuum along its length with unique electrical behavior. These structures are synthesized via the “nanotomy” process (developed by our group) and the devices are fabricated using electron beam lithography. Further, devices were also fabricated via non-nanotomy process through electron beam lithography with Pd-Au electrode-deposition. We demonstrate the generation of Schottky internal barrier and diodic transistor transport in these novel graphene structures.
2:45 PM - P7.05
Graphene-nanoribbon-based Heterojunctions as Components in Advanced Electronics
Graphene-based materials are promising candidates for a variety of applications in electronics. Graphene-based active layers in field-effect transistors are already making use of its excellent charge transport properties. However, before graphene can be deployed on a wide scale as the active electronic component in carbon-based integrated circuits, it will be critical to design and engineer heterojunctions with tunable, tailored functionality. Heterojunctions form the active components in many solid-state device applications. Therefore, in this study we use first-principles methods to explore the formation and properties of graphene-nanoribbon based heterojunctions for use in electronic componentry. Previous theoretical and experimental work indicates that, due to quantum confinement effects, armchair graphene nanoribbons are semi-conducting with an energy-gap scaling with the inverse of the GNR width. We use first-principles total-energy electronic-structure methods based on density functional theory to investigate the formation of periodic semiconductor heterojunctions (superlattices) composed of graphene nanoribbons of different width, connected by a smooth, continuous transition region. Our study reveals a number of unique properties of the heterojunctions. We establish that the interfacial transition regions must follow strict geometric rules with respect to compatible ribbon widths and tapering angles at the interfaces to avoid the formation of electronic states at the Fermi level that render the whole system to be metallic. When the semiconducting nature is preserved by a proper transition region, we find that our heterojunctions exhibit a type-I band alignment, in which the valence band maximum (VBM) and conduction band minimum (CBM) are localized to the wider regions of the heterostructured ribbon. In some cases when the lengths of these ribbons are appropriately engineered, we observe delocalization of the VBM and CBM to the entirety of the heterojunction, corresponding to the onset of tunneling. Further, varying the lengths of the two component nanoribbons in the heterojunctions introduces additional quantum confinement, which enables tuning of barrier and well size, and therefore offers additional degrees of freedom for tuning of band gaps. Our DFT predictions for the onset of tunneling (in which the VBM and CBM become delocalized) are in good agreement with a one-dimensional model to the Schrodinger equation invoking the envelope function approximation for superlattices. Finally, we also explore the effects of applying tensile and compressive strain on the quantum confinement and therefore the band gaps of the nanoribbons.
3:00 PM -
3:30 PM - *P7.06
Fluorination of Graphene: Synthesis, Properties and Challenges
Fluorinated carbon materials are useful in a wide range of applications from batteries to catalysis. A current direction in the field of graphene research explores the potential of fluorinated graphene (FG) as ultrathin gate dielectric and tunnel barrier in carbon electronics. In this talk, I will describe our effort in synthesizing and understanding the properties of FG. We have taken two complementary paths to the synthesis of FG. In the first approach, we utilize established synthesis of bulk graphite fluoride and obtain FG through the exfoliation of high-quality stoichiometric graphite monofluoride (GMF) crystals. I will present structural, optical and electrical transport studies of exfoliated FG. We observe, for the first time, UV Raman active modes of GMF. Photoluminescence measurements show several emission modes from near IR to near UV, which are likely due to defects in GMF. Electrical transport measurements confirm the strongly insulating nature of this compound although the size of the band gap remains elusive. In the second approach, we fluorinate graphene sheets synthesized via chemical vapor deposition (CVD) using CF4 plasma. The resulting FG is systematically studied using a wide range of spectroscopic and microscopic probes. We find that the structural features of CVD graphene (wrinkles, folds, multi-layer patches) play a critical role in the spatial distribution of fluorine and dominate the charge transport of FG. XPS studies reveal the complexity and evolution of the carbon-fluorine bonds, where the defects and grain boundaries of CVD graphene are important. These studies highlight current challenges in realizing electronics-grade FG and point to the possible pathways forward.
In collaboration with: Bei Wang, Shih-Ho Cheng, Justin Sparks, Humberto Gutierrez, Ke Zou, Junjie Wang, Ning Shen, Qingzhen Hao, Youjian Tang, Peter Eklund, Vincent Crespi, Jorge Sofo (Penn State University) and Fujio Okino (Shinshu University, Japan).
4:00 PM - P7.07
Chemically-modified, Twisted Bilayer Graphene: A Tunable Platform for Functionalization and Intercalation Studies
Bilayer graphene is a diverse material system that can be extended by the introduction of an interlayer misorientation angle, the “twist angle” (θ). By varying θ, a range of electronic and optical features can be tailored in electronically coupled, twisted bilayer graphene (TBG).[1-2] TBG properties continuously vary from those of Bernal stacked (θ=0°) bilayer graphene as the twist angle is increased. Extending this idea, we demonstrate the influence of θ on the chemical properties of TBG. We fabricate large-area, electronically coupled TBG films in a multi-step transfer process starting from graphene grown by low-pressure CVD on Cu foil. Using optical microscopy and Raman spectroscopy, we classify TBG domains into one of five twist angle categories, where individual TBG domains range in size from ~10 - 100 μm. The influence of fluorine and oxygen functional groups on these TBG films is subsequently characterized as a function of twist angle. As a control for non-selective defect introduction, we use ion-implantation to uniformly introduce vacancy-type defects into TBG films.
Following these treatments, Raman spectroscopy is used to probe both the defect structure of TBG and the influence of each process on interlayer coupling, which expresses a unique resonance signature in Raman spectra. Due to the large domain size, together with the optical contrast provided by TBG, optical microscopy is a useful and efficient technique for characterizing the influence of twist angle on each process. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) provide further insight into the structure and bonding configuration of each system. Finally, we take advantage of the optical contrast provided by TBG to explore the intercalation of fluorine into artificial few-layer graphene structures. For a range of conditions, we find fluorination with XeF2 at room temperature is restricted to the outermost surface of few-layer graphene, while subsurface layers remain electronically coupled and chemically pristine.
 G. Li et al., Nat. Phys. 6, 109-113 (2010)
 R.W. Havener et al., Nano Lett. 12, 3162-3167 (2012)
 J.T. Robinson, S.W. Schmucker, et al., submitted (2012)
4:15 PM - P7.08
A consequence of doping a two-dimensional material like graphene is that the dopants are fully exposed to the atmosphere and the usual mechanisms for corrosion protection are not applicable. Here we show how elements are incorporated into graphene and explain how these reactive surface atoms remain non-oxidized even when stored in air. In the present work, annular dark-field (ADF) imaging in a scanning transmission electron microscope is used to locate and identify the impurity atoms in the graphene lattice. Electron energy-loss spectroscopy (EELS) is also used to confirm the impurity atom identification. We have found that silicon, chromium, iron, cobalt, nickel, and copper impurity atoms on graphene remain in elemental form even when the doped graphene is exposed to air for extended times. With this information, first principles calculations explain these observations are due to preferential bonding of O to non-incorporated atoms and H passivation effects. While we have only examined the microstructure and have not explored catalytic potential of impurities incorporated in graphene, it is anticipated that inert graphene can be transformed to a very active catalyst by embedding metal clusters and individual atoms in defects in graphene. This phenomenon can be used to employ the catalytic, electronic, and magnetic properties of more elements than are usually available for unprotected materials in air or other corrosive environments.*
*The experimental work was done by MFC and GD and was supported by the Materials Sciences and Engineering Division of the Office of Basic Energy Sciences, U.S. Dept. of Energy. The calculations were performed by WW sponsored by NSF Award Number DMR-0925529 and the Center for Emergent Materials at The Ohio State University, a NSF MRSEC (Grant DMR-0820414). W.W. also acknowledges support from the Ohio Supercomputer Center under project PAS0072.
4:30 PM - P7.09
Large-area Chemically Doped Graphene Sheets by Ambient-pressure Chemical Vapor Deposition
Li, Andres R.
Feng, Ana Laura
Terrones, Jean Christophe
Chemical doping is an efficient way to tailor the electronic, chemical and magnetic properties of graphene-like materials. We now report the synthesis of large-area, high-quality monolayer graphene sheets doped with different heteroatoms (N, B, Si, etc) on copper foils by a modified ambient-pressure chemical vapor deposition (AP-CVD) route. When compared to pristine graphene, the Raman spectra of heteroatom-doped graphenes show strong D-band caused by presence of non-carbon atoms and other structural defects contained within the lattice. In addition, significant changes in the intensity of the D’-band were noted. Scanning tunneling microscopy (STM) and spectroscopy (STS) studies reveal that the defects in the doped graphene samples arrange in different geometrical configurations. For N-doped graphene, we observed dominant “peapod-like” features corresponding to double N substitution in the same graphene sublattice (N2AA). For the B-doped graphene sample, novel “croissant-like” features were detected, which could be attributed to B3-type doping configurations embedded in the hexagonal honeycomb lattice. These experimental results are in agreement with first principles calculations of local density of states (LDOS) of doped graphene and STM image simulations.
4:45 PM - P7.10
Correlated Photocurrent and Fluorescence Imaging from Individual Graphene Oxide Nanosheet
Graphene oxide (GO) is an attractive material as a precursor to graphene. Structurally, GO is a graphene sheet with oxygen-containing functional groups which are present in the form of carboxyl, hydroxyl and epoxy groups. GO is an electrical insulator due to disrupted conjugation of sp2 bonds in the basal plane but can be controllably converted to a conductor via reduction. The fundamental limit of GO as an electronic material is that the functional groups cannot be removed completely, thus leaving a highly defective material with carrier mobilities that are orders of magnitude lower in comparison to exfoliated or even CVD graphene. Despite its high degree of disorder, however, GO exhibits intriguing and unique properties that arise from “states” induced by the oxygen functional groups. One example is a tunable photoluminescence (PL), which can be tuned from blue to red by progressive chemical reduction. Our previous PL measurements have shown that the states at the boundary of sp2 and sp3 domains play a critical role. Here, using correlated scanning photocurrent microscopy (SPCM) and fluorescence with a diffraction limited spatial resolution, we report the optoelectronic properties of an individual GO flake as a function of controlled reduction. Progressive reduction of GO allows us to precisely control the lateral density of the sp2 (graphitic) and sp3 (oxygen bearing) domains. This provides a strong handle in monitoring the evolution of photocurrent and understanding the role of oxygen bearing functional groups that are involved in the PL. Furthermore, electric force microscopy (EFM) was used to investigate the of charge migration within the single nanosheet and to map the local potentials as we transition from GO to reduced GO (RGO).
P8: Poster Session: Doping and Applications
- Wednesday PM, April 3, 2013
- Marriott Marquis, Yerba Buena Level, Salons 7-8-9
8:00 PM - P8.01
Effect of SAMs-layer on Optical Response of CVD Graphene: A Study toward Optoelectronic Device Application
Kim, Joo Youn
Lee, Kwang Nam
Yu, Yong Seok
Yoon, Eun Jip
Choi, Sang Jin
Kim, Byung Hee
Hong, Beom Joon
Kim, Jeong Ho
We report here optical transmission spectra of CVD graphene integrated with Self Assembled Monolayers (SAMs) over wide frequency range from Far-IR to UV (4meV-6.2eV). Monolayers with different functional groups --CH3, NH2 and HMDS-- were fabricated on SiO2/Si and Al203 substrates by chemical reaction and then CVD-grown graphene was transferred on the SAMs-modified substrates into the Graphene/SAMs/Substrate structure.
In the Far-IR range, Drude absorption is observed from which we discuss change of plasma frequency, scattering rate, and their implication on the mobility of free carrier. In the Mid-IR range, interband absorption and Fermi level shift (Pauli-blocking) are controlled by the characteristics of SAMs. In ultraviolet-visible range, the excitionic absorption peak (at 4.6eV) exhibits changes in the peak position and strength depending on the SAMs-layers. Our result shows that optical response of CVD graphene can be modified by the insertion of the SAM-layers in wide frequency range which can be used in large scale optoelectronic device application.
8:00 PM - P8.02
Graphene Oxides: Tunable Broadband Nonlinear Optical Materials for Femtosecond Laser Pulses
Nonlinear optical materials (optical limiters and saturable absorbers) play a significant role in the field of optics by their unprecedented ability to modulate lasers pulses, as lasers have become an integral part of our lives with various potential applications. Graphene oxide (GO) thin films were found to display interesting broadband nonlinear optical properties. Their optical limiting activity for femtosecond laser pulses at 800 and 400 nm was investigated, which could be tuned by controlling the extent of reduction. The as-prepared GO films were found to exhibit strong broadband optical limiting behaviors, which were attributed to the nonlinear absorption (two-photon or three-photon absorption) of GO films. Their optical limiting performances were found to be significantly enhanced upon partial reduction by laser irradiation or chemical methods. The laser induced partial reduction of GO could result in enhancement of effective two-photon absorption coefficient at 400 nm by up to ~19 times, and enhancement of effective two- and three-photon absorption coefficients at 800 nm by ~12 and ~14.5 times respectively. The optical limiting thresholds of partially reduced GO films are much lower than those of various previously reported materials. Highly reduced GO films prepared by using chemical method displayed strong saturable absorption behavior. These thin films could be easily fabricated on glass and even plastic substrates by using solution processing methods on a large scale. Additionally, the good solubility of GO sheets enables them to easily mix with polyvinyl alcohol (PVA) to produce the flexible free-standing films. During the subsequent in-situ reduction of GO, the polymer matrix can prevent the re-aggregation of reduced GO sheets to retain a homogeneous suspension. Their nonlinear optical properties of femtosecond laser are well consist with the GO thin film on substrates. Low cost, easy preparation and excellent nonlinear optical properties make these GO materials promising candidates for practical applications as broadband femtosecond optical limiters or saturable absorbers.
8:00 PM - P8.03
Reduced Graphene Oxide Film for Transparent and Flexible Optoelectronics
Lee, Jin Ok
Hwang, Ji Sun
Park, Joon Won
Lim, Sang Ouk
We have demonstrated a novel transparent and flexible optoelectronic hybrid material based on reduced graphene substrates. The hybrid material created from the hydrothermal synthesis of ZnO nanowires on reduced graphene/PDMS substrates proved to be a promising candidate for flexible devices, such as flexible field emission devices. By Owing to the good mechanical and electrical contact between vertical ZnO nanowires and graphene film, the field emission of our hybrid material showed low turn-on voltages from 2.0 to 2.8 V µm-1, even in highly deformed geometries. Furthermore, the optimal doping of quaternary nitrogen via sequential hydrazine treatment and thermal reduction accomplished workfunction-tunable reduced graphene oxide film. The PLEDs employing N-doped reduced graphene cathodes exhibited a 7.0 cd/A electroluminescence efficiency for N-doped graphene at 17 000 cd/m2. Reduced barrier for electron injection from a workfunction-tunable, N-doped reduced graphene cathode offered remarkable device performance. We anticipate that the mechanical deformability and interesting electric and optoelectronic properties of graphene hybrid device would provide valuable insights for flexible applications, such as flexible displays and solar cells.
8:00 PM - P8.04
Single-walled Carbon Nanotube Film as an Conductive Support for Flexible Electrochemical Energy Storage Devices
Transition metal oxide (MOx), such as MnO2, Co3O4, etc., has been proved to be promising candidates in electrochemical energy storage fields. However, the poor electrical and ionic conductivities and low stability of MOx hinder its applications. Forming composites with high surface area porous metal, carbon materials, or conducting polymers is a possible solution. Herein, we have developed facile and scalable asymmetric in situ deposition methods to incorporate MOx nanoparticles into conductive single-walled carbon nanotube (SWCNT) films. The high porosity of the SWCNT films accommodates MOx nanoparticles without sacrificing the mechanical flexibility and electrochemical stability of SWCNT films.
For example, we exposed one side of SWCNT films to acidic potassium permanganate (KMnO4) solution. The infiltrated KMnO4 solution partially etches SWCNTs to create abundant mesopores, which ensure electrolyte ions efficiently access deposited MnO2. Meanwhile, the remaining SWCNT network serves as excellent current collectors. The electrochemical performance of the SWCNT-MnO2 composite electrodes depends on the porosity of SWCNT films, pH, and concentration of KMnO4 solution, deposition temperature and time. Our optimized two-electrode electrochemical capacitor, with 1 M Na2SO4 in water as electrolyte, showed a superior performance with specific capacitance of 529.8 F g-1, energy density of 73.6 Wh kg-1, power density of 14.6 kW kg-1, excellent capacitance retention (99.9%) after 2000 charge and discharge cycles, and one of the highest reported frequency responses (knee frequency at 1318 Hz).
In another case, we first deposited the precursor of Co3O4 nanoparticles at the surface of SWCNTs. After calcination, necklace-like SWCNT-Co3O4 composite was produced. The conductive SWCNT matrix can efficiently reduce the resistance of the battery when the composite material is used as anode in lithium ion battery (LIB). The performance of the SWCNT-Co3O4 electrodes mainly depends on the mass loading of Co3O4 in the composite, reaction temperature and time. The specific capacity of the LIB equipped with the SWCNT-Co3O4 composite electrode reaches 1239 mAh/g at 0.1C, 750 mAh/g at 1C, and 180 mAh/g at 10C, respectively, and keeps 90.5%, 89.5%, and 83.3% of its original capacity at 1C, 5C and 10C after 1000 charge and discharge cycles.
The high performance flexible SWCNT-MOx composites have broad applications in portable electronics and electrical vehicles, especially where both high power and energy densities, and high frequency response are desired.
8:00 PM - P8.05
Functionalized Hierarchical Vertically-oriented Graphene as a Catalytic Counter Electrode in Dye-sensitized Solar Cells: Negligible Charge Transfer Resistance
Dye-sensitized solar cells (DSSCs) represent a viable alternative to silicon-based solar cells due to their potential low cost, easy fabrication, and high efficiency (over 12%). In a typical DSSC operation, the iodide (I-) in electrolyte is oxidized at the dye-sensitized porous nanocrystalline TiO2 under illumination, while the triiodide (I3-) is reduced at the counter electrode (CE) for the purpose of dye regeneration. The counter electrode usually consists of a transparent conductive oxide (TCO) layer deposited with a layer of catalyst. Platinum is often selected for use as the catalyst because it has high catalytic activity toward I3- reduction and is sufficiently corrosion-resistant to iodine species present in the electrolyte. However, since platinum is a highly valued metal, much incentive exists to develop DSSC counter electrodes using cheaper, abundant materials that are equal or greater in their catalytic activity.
Carbon nanomaterials have been studied as a replacement counter electrode for platinum. Manipulating these nanostructures also allows for the control and use of defect-rich edge plane materials, which facilitate the electron kinetics associated with the reduction of I3-. Particularly, graphene has emerged as a potential catalyst for DSSC cathode, due to graphene’s exceptional surface area and conductivity.
This study examines the use of vertical graphene (VG) as a CE of DSSCs. Based on our early research on VG, this material has many advantages over planer graphene, such as easy fabrication, highly porous structure, and abundant surface oxygen functional groups. The catalytic activity of VG for the reduction of I3− is studied using electrochemical techniques and density functional theory (DFT) calculations. By introducing different amounts of water vapor during the synthesis, the content of oxygen functional groups on VG can be tuned, which eventually leads to varying catalytic performance in DSSCs. The DFT calculation reveals that the oxygen functional groups play key roles for the I3− reduction. The VG electrode achieved a charge transfer resistance (RCT) of 7.3 × 10-3 Ω cm2 versus 0.59 Ω cm2 for platinized FTO glass (Pt/FTO). To the best of our knowledge, this is the lowest RCT for I−/I3− redox couple of a DSSC CE. DSSCs fabricated with VG as CEs exhibit superior performance over that with Pt.
8:00 PM - P8.06
Graphene as a Protective Layer for Silicon Photoanodes in an Aqueous Photoelectrochemical Cell
Hydride-terminated Si(111) photoelectrodes form a deleterious SiOx layer when anodic current is passed from the silicon electrode to an aqueous solution. This prohibits the use of silicon as a photoanode in a viable water splitting device. Graphene grown via chemical vapor deposition (CVD) was used to fabricate planar (111) n-type silicon photoanodes covered by monolayer graphene. Graphene-covered, H-terminated n-Si electrodes in contact with an aqueous K3[Fe(CN)¬6]/K4[Fe(CN)¬6] electrolyte exhibited increased stability under illumination. The short-circuit current (Jsc), open-circuit voltage (Voc), and fill factor (ff) decayed significantly less slowly for graphene-covered n-Si than for the corresponding bare H-terminated n-Si. The short-circuit current of the n-Si-H decayed to half its initial value in ~30 seconds while the same 50% decay in the short-circuit current for graphene-covered n-Si took ~75,000s. This behavior was attributed to a reduced rate of oxidation of the silicon surface when the silicon was covered by graphene. In non-aqueous electrolytes, graphene-covered n-Si electrodes demonstrated reduced Voc when compared to graphene-free H-terminated n-Si electrodes. Studies of Voc vs. solution redox potential for graphene-covered n-Si electrodes exhibited changes in Voc concomitant with redox solution potential. This Voc vs. solution potential relationship demonstrates that neither complete Fermi level pinning by graphene nor silicon bulk diffusion current limits the observed voltages.
8:00 PM - P8.07
Highly Efficient Polymer Light-emitting Diodes Using Graphene Oxide-modified Flexible Single-walled Carbon Nanotube Electrodes
Song, Joong Tark
Han, Bo Ram
We present flexible polymer light-emitting diodes (FPLEDs) using graphene oxide-modified single-walled carbon nanotube (GO-SWCNT) films as an anode on polyethylene terephthalate (PET) substrates. The electrode of GO-SWCNTs used in this study shows a quite low sheet resistance of ~75 Ω/square at 65 % (at 550 nm) optical transparency with resistance to bending fatigue. The small-sized GO nanosheets onto the SWCNT network films are significantly effective in reducing the sheet resistance and the surface porosity of the SWCNT network films without sacrificing the transmittance. Moreover, the top layer of the large-sized GO nanosheets are crucial for high device efficiency to reduce the roughness of the SWCNT surface and enhance the wettability with PEDOT:PSS. The optimized FPLED using a GO-SWCNT electrode shows a maximum luminous efficiency of 5.0 cd/A (at 9.2 V), power efficiency of 2.4 lm/W (at 5.6 V), external quantum efficiency 1.9 % (at 9.0 V) and turn-on voltage (2.0 V), which is comparable to conventional PLEDs using an indium-tin-oxide (ITO) electrode (a maximum luminous efficiency of 6.2 cd/A (at 9.4 V), power efficiency of 2.6 lm/W (at 5.8 V), external quantum efficiency 2.3 % (at 9.0 V) and turn-on voltage (2.0 V)). This result confirms that a GO-SWCNT electrode can be efficiently used to replace ITO for flexible optoelectronic devices.
8:00 PM - P8.08
Covalent Chemical Modification of Multiwalled Carbon Nanotubes Using Azide Functionalised Anthraquinone Derivatives for Pseudocapacitor Application
Le Barny.Show Abstract
Chemical modification of carbon nanotubes (CNTs) is being the focus of much research, either to improve their solubility in various solvents or to add a functionality. Until now, most of the work dedicated to energy storage, with this approach, has been focussed on the development of pseudocapacitors based on activated carbon modified by a diazonium derivative of the electroactive anthraquinone (AQ) group. Even though activated carbon is currently the state of the art electrode material, it suffers from some drawbacks including its limited electrical conductivity and the need for a binder to ensure its expected cohesion. These drawbacks can be overcome when CNTs are used instead of activated carbon. Indeed, CNTs can be easily transformed into a nanotube network (bucky paper) by filtration of a solvent nanotube suspension leading to a self-supporting electrode with quite good electrical conductivity.
In this work, we report the preparation and the characterization of electrodes made of multiwalled carbon nanotubes (MWCNTs) chemically modified by a series of AQ derivatives. The covalent attachment of the AQ moiety to MWCNTs was performed via an azido group attached either to an alkyl spacer or to an aryl group, thus allowing to study the influence of the distance between the AQ moiety and the MWCNTs on the electrochemical properties of the resulting electrode. The modified MWCNTs (AQ-MWCNT) were characterized by thermogravimetric analysis and X-ray photoemission. Bucky papers obtained by filtration of a aprotic polar solvent AQ-MWCNT suspension, were characterized by cyclic voltammetry in 0.1M H2SO4, using the standard 3 electrode set-up. Thus, it has been demonstrated that a MWCNT electrode modified by an aryl azide AQ-MWCNT having a eight methylene groups spacer provides 54% higher specific capacitance, compared to a unmodified MWCNT electrode.
8:00 PM - P8.09
Enhanced Electron Emission Characteristics from Metal Nanoparticles Decorated Multilayer Graphene Nanocomposites
Gaphene is a “new rising star” in carbon-based materials due to its extraordinary properties at the nanoscale, such as high charge carrier mobility, electrical and mechanical properties. Now a days, there has been tremendous interest in graphene-metal nanocomposite materials because of their potential applications in field emission display devices, microelectronics, hydrogen sensing, high catalytic activity etc. In the present work metal (Pd, Ag, Au & Ti) nanoparticles are grafted by thermal evaporation and multilayer graphene (MLG) synthesized by microwave plasma enhanced chemical vapour deposition system, to form metal-MLG nanocomposites. A scanning and transmission electron microscopy study shows beautiful decoration of these metal nanoparticles of diameter 5 - 20 nm on graphene layers. During Raman spectroscopy studies, it was found that metal-MLG nanocomposites emit enhanced resonance in comparison with the as-deposited MLG and is related to surface enhanced Raman spectroscopy. The ID/IG ratio was found to decrease and is related to the passivation of defects, due to which the field emission is enhanced. Field emission studies of metal-MLG nanocomposites were carried out using diode type field emission set-up. Metal grafted MLG nanocomposites have low work function and higher field enhancement factor as compared to as deposited MLG. The enhancement in the field emission properties of metal grafted MLG nanocomposites is considered to be due to the modification in density of states near Fermi level. To investigate the microscopic origin for the enhanced field emission from metal grafted MLG, we carried out the first principles calculations using Mede A software. We investigate the density of states of these metal-MLG nanocomposites. It is found that the DOS are increased near the Fermi level after the decoration of metal nanoparticles over graphene. The theoretical results are found to be consistent with the observed experimental data.
8:00 PM - P8.11
Fabrication and Characterization of Graphene p-n Vertical Junctions for Different n Doping Concentrations
Shin, Chang Oh
Kim, Chan Wook
Jang, Ju Hwan
Kim, Jong Min
Single-layer graphene was synthesized by using chemical vapor deposition, and transferred on SiO2/Si substrates. For the formation of graphene p-n junction, a solution of benzyl viologen (BV) was first dropped and spin-coated on the 10 x 10 mm2 graphene/SiO2/p-type Si wafer, and then annealed at 100 oC for 10 min to make graphene uniformly n-type. Subsequently, a 5 x 5 mm2 bare graphene was transferred on ~1/4 area of the n-graphene/SiO2/p-type Si wafer, a solution of AuCl3 was dropped and spin-coated on the surface of graphene, and similarly annealed. As a result, the graphene p-n vertical homojunction was formed on the ~1/4 area of the SiO2/p-type Si wafer. 1-mm-diameter Ag electrodes were deposited on the top of both n- and p-graphene layers to complete the graphene p-n device. We prepared the graphene p-n vertical-homojunctions for various n doping concentrations at a fixed highest p-doping concentration. At lower n doping concentrations, the p-n junctions are ohmic, consistent with the Klein-tunneling effect. In contrast, at higher n doping concentrations, the p-n junctions show asymmetric rectifying behaviors. These results are discussed based on possible physical mechanisms.
8:00 PM - P8.12
Amine Organo-functionalized Carbon Nanotubes as Nano-reinforcing Agents of Epoxy Polymers
The outstanding properties of carbon nanotubes (CNTs) and the steps forward on their controllable growth and surface functionalization, have boost research interest for their use as nano-additives in the development of novel polymer nanocomposites. Epoxy resins represent one of the most important classes of engineering polymers, with unique properties and wide variety of applications. The incorporation of carbon nanotubes in epoxy polymers can lead to significant improvement of their mechanical, thermal and conductivity properties. The successful preparation of nanocomposites strongly depends on the homogenous dispersion and interfacial interactions of CNTs with the polymer matrix. In order to promote those critical aspects organic modification and surface functionalization of CNTs is necessary.
Significant progress has been achieved so far in the development of nanocomposites using glassy epoxy polymers and organically modified carbon. In this study, we focus on the development of rubbery epoxy polymers reinforced with various types of multi wall carbon nanotubes, i.e. having varying length and width, being also surface modified with carboxyl groups (-COOH), various alkyl-amines (CxHy-NH2) or polyoxypropylene amines (Jeffamine D-2000). The effect of mixing conditions and percentage loading of CNTs were also investigated. The in-situ polymerization technique was used for the production of the epoxy-CNTs nanocomposites.
The characterization results of CNTs revealed a minor effect of chemical modification treatment on nanotubes’ structural integrity and the successful grafting of the various functional organic groups. The mechanical properties of the epoxy polymer were significantly improved especially by the use of hexamethyl amine and polyoxypropylene D-2000 diamine functionalized MWCNTs. The respective nanocomposites exhibited simultaneous increase of stiffness and toughness. Short and thin carbon nanotubes also favor both the above properties to higher extent compared to larger size MWCNTs. The viscoelastic properties, such as storage modulus and glass transition temperature (Tg), were also improved with most of the CNT variants used as nano-additives, whereas, the thermal stability of the epoxy polymer was not sacrificed.
ACKNOWLEDGEMENTS. Co-funding of this research by EU-ESF and the Greek Ministry of Education, Lifelong Learning and Religious Affairs via the Program “Education and Lifelong Learning”, Action “Support of Postdoctoral Researchers/ESPA 2007-2013” is gratefully acknowledged.
8:00 PM - P8.13
Defect Dependent, Trion Fluorescence of Single-walled Carbon Nanotubes
Near-IR fluorescence is one of the unique properties of single-walled carbon nanotubes (SWCNTs) that has attracted significant attention for biomedical and optoelectronic applications. However, modification of SWCNTs by covalent functionalization often completely destroys exciton photoluminescence due to the introduction of sp3 defects that disrupt the pi-conjugated system of the pristine, sp2 carbon lattice. Here, we report evidence of photoluminescence from trions, or charged excitons, that may form in covalently functionalized SWCNTs as mobile excitons meet localized electrons or holes at sp3 defect sites. Two distinctly different chemistries, including the reductive Billups-Birch reaction, which injects excess electrons to the covalently functionalized SWCNTs, and oxidative, super-acid chemistry, which protonates the SWCNTs, produce similar trion photoluminescence features. This new trion emission is red-shifted by approximately 260 meV and is stable under ambient conditions, presumably due to the ability of sp3 defects to trap and stabilize the excess carrier population that enables charged excitons to form. Thin film transistors of the functionalized SWCNTs confirm the doping conditions which promote trion formation.
8:00 PM - P8.14
Efficient p-type Doping of Graphene Electrodes via Transition Metal Oxides for Organic Light Emitting Diodes
Electrode materials combining high electrical conductivity and optical transparency are crucial components for organic light emitting diodes (OLED). Graphene is thereby a highly promising alternative to commonly used Indium tin oxide (ITO), in particular considering that unlike ITO graphene is flexible and, when grown via Chemical Vapor Deposition (CVD), not constraint by limited natural resources. Critical challenges for graphene based OLEDs not only relate to the further improvement of large-area, controlled graphene CVD [1,2], but also to its integration, in particular to achieve efficient charge injection and graphene doping. Several dopants have recently been introduced, but most are chemically not stable or not applicable for organic electronic devices.
Here we show that transition metal oxides, such as MoO3 and WO3, are very efficient and stable p-type dopants for graphene leading to a more than three-fold reduction of sheet resistance. Our process is based on scalable graphene CVD [1,2] and the doping is carried out via thermal evaporation which can be fully integrated in the OLED fabrication process. With in-situ 4-point probe measurements we find that only a few nanometers of MoO3 are sufficient for efficient doping. Our systematic study of this doping process by ultra-violet and x-ray photoemission spectroscopy (UPS, XPS) shows a large interface dipole of 2.2 eV and band bending caused by an electron transfer from graphene to MoO3. The strong p-doping of graphene is enforced by the deep lying electronic states of MoO3 which exhibits a work function of >6.5 eV. Our XPS analysis also shows that the doping process leads to a metal oxide reduction from Mo 6+ to Mo 5+ states at the interface. The energy level alignment at the graphene/MoO3/organic interfaces as measured with UPS shows only very small energy barriers for hole-injection. Thus, MoO3 allows not only an efficient p-doping of graphene, but also provides a suitable matching for efficient hole-injection from graphene into the OLED layers. Based on this process, we demonstrate CVD graphene based OLEDs that show electro-optical performances similar to conventional ITO based devices.
Authors acknowledge funding from the EC project Grafol
 Kidambi et al., J Phys Chem C (2012) DOI 10.1021/jp303597m
 Weatherup et al., ACS Nano (2012) DOI 10.1021/nn303674g
8:00 PM - P8.15
Doped Graphene Catalytic Support for Fuel Cell Application
In fuel cell technology, the development of efficient catalysts and method for catalyst deposition is crucial. Indeed, the efficiency of the catalyst will control the kinetics of the reaction by decreasing the activation energy. The activity and durability can be improved by optimizing the carbon support of the active catalyst[1,2]. A catalyst widely used in fuel cell applications is platinum (Pt), which is responsible for the cost of the fuel cell system. Our research focuses on improving the catalytic efficiency and durability of Platinum while reducing the amount needed for the development of a reliable fuel cell system. This goal is achieved by using carbon surfaces with very high surface area and by modifying the local electronic distribution of the surface using dopants. Therefore understanding and controlling the deposition and catalytic activity of the platinum is essential to develop a viable and reliable system. The paper will present the effect of dopant on the durability and efficiency of the catalyst for the oxygen reduction reaction, which represents the rate limiting factor in a proton exchange membrane fuel cell. This study was done using structural and dynamic ab initio methods.
 M. N. Groves, C. Malardier-Jugroot, M. Jugroot. J. Phys. Chem. C, 116 (19), 10548, 2012
 M. N. Groves, A.S.W. Chan, C. Malardier-Jugroot, M. Jugroot, Chemical Physics Letters, 481, 4-6, 2009
8:00 PM - P8.17
The Impact of Functionalization on the Stability, Work Function and Photoluminescence of Reduced Graphene Oxide
Reduced graphene oxide (rGO) is a promising solution processable material for a variety of thin-film optoelectronic applications. The two main barriers to widespread adoption of rGO are the lack of (1) fabrication protocols leading to tailored functionalization of the graphene sheet with oxygen-containing chemical groups, and (2) understanding of the impact of such functional groups on the optical properties and electronic structure of rGO. We carry out classical molecular dynamics and ab initio density functional theory calculations on a very large statistical set of disordered rGO structures to demonstrate the role of functional groups on the stability, work function and photoluminescence of rGO. Our calculations indicate the metastable nature of carbonyl-rich rGO structures at room temperature, and show the favorable energetics for their conversion to hydroxyl-rich structures via carbonyl to hydroxyl conversion near carbon vacancies and holes. We demonstrate a significant tunability in the work function of rGO by up to 0.5 eV by changing the oxygen content and by up to 2.5 eV in structures with a precise control of oxygen-containing functional groups. We show that the PL emission peak in rGO can be fine-tuned by modulating the fraction of epoxy and carbonyl groups. Taken together, our results guide the preparation of controlled rGO structures and pave the way to their application in the fields of optoelectronics and energy conversion.
8:00 PM - P8.18
Direct Growth of Graphene Micro Ribbons on Dielectric Substrates by Controlling Nickel Dewet Region
Here we report on a scalable and direct growth of graphene micro ribbons on SiO2 dielectric substrates using a low temperature chemical vapor deposition. Due to the fast annealing at low temperature (750oC for 2-5 minutes) and dewetting of Ni, continuous few-layer graphene micro ribbons (2-10 µm in width, up to a few millimeters in length) grow directly on bare dielectric substrates through Ni assisted catalytic decomposition of hydrocarbon precursors. These high quality graphene micro ribbons exhibit low sheet resistance of ~700 Ω -2100 Ω, high on/off current ratio of ~3, and high carrier mobility of ~655 cm2V-1s-1 at room temperature, all of which have shown significant improvement over other lithography patterned CVD graphene micro ribbons. This direct approach can in principle form graphene ribbons of any arbitrary sizes and geometries. It allows for a feasible methodology towards better integration with semiconductor materials for interconnect electronics and scalable production for graphene based electronic and optoelectronic applications where the electrical gating is the key enabling factor.
8:00 PM - P8.19
Characterizations of HOPG and Graphene Treated with Low Temperature Hydrogen Plasma
Single and multilayer graphene and highly ordered pyrolytic graphite (HOPG) were exposed to pure hydrogen low temperature plasma (LTP).
In the first part, hydrogen LTP exposed HOPG is characterized with various experimental techniques such as photoelectron spectroscopy, Raman spectroscopy and scanning probe microscopy. Our photoemission measurement shows that hydrogen LTP exposed HOPG has a diamond-like valence band structure, which anticipates graphane formation. With the scanning tunneling microscopy technique, various atomic scale charge density patterns were observed, which might be associated with different C-H conformers of graphane. A very low defect density was observed by the scanning probe microscopy measurements, which enables a reverse transformation to graphene. Hydrogen LTP exposed HOPG possesses a high thermal stability, and therefore, this transformation requires annealing over 1000oC.
In the second part, a silicon stencil mask with a periodic pattern is used for hydrogen plasma microlithography of graphene supported on a Si/SiO2 substrate. Obtained patterns are imaged with Raman microscopy and Kelvin probe force microscopy, thanks to the changes in the vibrational modes and the contact potential difference (CPD) of graphene after treatment. A decrease of 60 meV in CPD as well as a significant change of the D/G ratio in the Raman spectra can be associated with a local hydrogenation of graphene, while the topography remains invariant to the plasma exposure.
8:00 PM - P8.20
Control of Edge Types of Epitaxial Graphene Nanoribbons Grown on Vicinal SiC Surfaces by Molecular Beam Epitaxy
Graphene nanoribbons (GNRs) attract great attentions in nano-electronic applications and solid state physics. Modification of electronic structures at K-point in GNRs has theoretically and experimentally been demonstrated: those are differed depending on the type of edge geometries. Armchair and zigzag edges indicate band-gap and a flat band at the K-points, respectively . So far it is difficult to fabricate such dense GNRs as having a sole type of edges to macroscopically visualize those electronic structures at the K-points, which can be achieved by angle-resolved photoemission spectroscopy (ARPES). We have reported the growth of GNRs, possessing armchair edges, on unique SiC surfaces by molecular beam epitaxy (MBE). The GNRs were grown on vicinal SiC surfaces, consisting of periodic (0001) terraces/(1-10n) facets induced after high temperature H2-gas etching . The massive arrays of GNRs, approximately 10 nm width and armchair edges, were obtained and characterized by polarized Raman spectroscopy and ARPES. Essentially those indicate apparent band-gap openings at the K-point of at least 0.14 eV. The edge type of the GNRs is determined by the vicinal direction of SiC substrate, which is [1-100] for armchair. Initially the buffer layer, which shows (6√3×6√3)R30 geometry to SiC(1×1), is epitaxially grown on SiC(0001) and as a result its edge should crystallographically be armchair type, as was confirmed by the polarized Raman spectroscopy . Thus, the edge type of GNRs can be controlled by the use of different vicinal directions of off-axis SiC substrates. We here demonstrate the growth of both armchair and zigzag GNRs based on this strategy and show electronic structures.
 K. Nakada et al., Phys. Rev. B 54, 17954 (1996).
 T. Kajiwara et al., submitted. arXiv:1210.4304v1. (2012)
 H. Nakagawa, S. Tanaka, and I. Suemune, Phys. Rev. Lett. 91, 226107 (2003).
 K. Sasaki et al., Phys. Rev. B 82, 205407 (2010).
8:00 PM - P8.23
Passivation and Non-covalent Functionalization of Graphene Using Self-assembly of Alkane-amines
As a “surface-only” nanomaterial, the properties of monolayer or few-layer graphene structures are extremely sensitive to adsorbed ambient contaminants, with a correspondingly severe impact on the electrical characteristics and stability of graphene-based devices. We report on a simple, versatile functionalization method based on solution-phase self-assembly of alkane-amine layers on graphene.
Second-order Moller-Plesset (MP2) perturbation theory on a cluster model (methylamine on pyrene) yield a binding energy ~ 220 meV for the amine-graphene interaction, strong enough to enable formation of a stable aminodecane layer at room temperature. The MP2 calculations also indicate a weak charge transfer (0.01 electrons) from the amine to the pyrene. Atomistic molecular dynamics simulations on an assembly of 1-aminodecane molecules indicate that a self-assembled monolayer can form, with the alkane chains oriented perpendicular to the graphene basal plane. The calculated monolayer height (~ 1.7 nm) is in good agreement with Atomic Force Microscopy data acquired for graphene functionalized with 1-aminodecane, which yield a continuous layer with mean thickness ~1.7 nm, albeit with some island defects.
Passivation and adsorbate of graphene field-effect devices using 1-aminodecane yields with carrier mobilities in excess of 4000 cm2/(Vs) for electrons and holes at field-induced carrier concentrations of 1x10^12 (cm)^-2. Device mobilities are maintained, even after re-exposure to atmospheric conditions for up to 6 months. Molecule-passivated devices also show low hysteresis and excellent stability in terms of the (net) doping level.
Raman data also confirm that self-assembly of alkane-amines is a non-covalent process, i.e., does not perturb the sp2-hybridization of the graphene.
Further attractive features of this functionalization method include high-density binding of plasmonic metal nanocrystals for enhanced photoconductivity and seeded atomic layer deposition of inorganic dielectrics.
8:00 PM - P8.24
Plasma Doping and Recovery on Mechanically Exfoliated Graphenes
Graphene has attracted much attention due to its remarkable physical properties and potential applications in many fields. In special, the electronic properties of graphene are influenced by the number of layer, stacking sequence, edge state, and doping of foreign elements. Recently, many efforts have been dedicated to alter the electronic properties by doping of various species, such as hydrogen, oxygen, nitrogen, ammonia and etc.
Here, we report our recent results of plasma doping on graphene and its recovery behavior. We prepared mechanically exfoliated graphene, which contains mono-, bi- and multi-layered regions, and performed the plasma treatment using air and ammonia gas for oxygen and nitrogen doping, respectively. To compare the doping efficiency, we also performed air annealing. The recovery of the doped samples was carried out by high vacuum annealing at elevated temperatures. The doping level was estimated from the number of peak shift of G-band in Raman spectra. The downshift of G-band was observed after ammonia plasma treatment, which implies electron doping to graphene. On the other hand, the upshift of G-band was measured as a result of thermal annealing in air, which means hole doping. After the high vacuum annealing, we confirmed the recovery of the doped graphenes by replacement of G-band position in Raman spectra. Finally, the reversible process of doping and recovery can be achievable through controlled doping and recovery treatments.
8:00 PM - P8.25
Chemical Doping of CVD-grown Graphene and Its Photoresponse
Graphene has been regarded as a promising material in electronics, and recently, it is a rising star in photonics as well. It shows strong and broadband absorption per unit mass extending across a wide range from visible to infrared (IR). Together with its extremely high mobility, it is very potential in ultrafast broadband photodetector. Moreover, the electronic property of graphene is largely controlled by the chemical potential, which requires controlled doping.
Here, we report the use of 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole (o-MeO-DMBI) as a strong n-type dopant for chemical-vapor-deposition (CVD) grown graphene. After doping, the CVD-grown graphene can be efficiently tuned from p-type to ambipolar and finally n-type transport behaviors, by simply varying the amount of o-MeO-DMBI applied. Also, if doped site-selectively, the CVD-grown graphene can be constructed to large amounts of p-n junctions which are composed by undoped and doped regions. By using photolithography, we can define the area ratio of p- and n- parts, which affects the ratio of hole-electron mobility. And by changing the amount of o-MeO-DMBI applied, we can tune the relative doping density between p- and n- parts, which affects the Fermi energy separations of p-n junctions. Photoresponse was tested on the FET devices which were fabricated by only n-doped graphene and p-n patterend one. Under a cold cathode fluorescent lamp, different photoresponses were observed for the only n-doped and p-n pattered graphene. Besides, the larger energy separations of p-n junctions lead to stronger photoresponse, which can be obtained from higher doping differences among the p- and n- parts. We also found that such photoresponse was attributed by junctions as well as bolometric effect. The chemical doping of CVD-grown graphene and its photoresponse demonstrated in this study is believed to be a feasible scheme in manipulating graphene chemical potential for future graphene-based optoelectroncis.
8:00 PM - P8.26
In situ Nitrogen-doped Graphene Grown from Polymers with Plasma Assistance
Since graphene grown from polymer by Sun et al. [Nature 468, 549, (2010)] firstly, using solid carbon source as potential feedstock for graphene synthesis has been proposed as a new environment benign approach. Unfortunately, due to the rapid evaporation of PMMA under high temperature, it was reported difficult to synthesize graphene converting from PMMA by conventional CVD. In this work, microwave plasmas were employed to synthesize single- or double-layer large scale graphene sheets on copper foils using solid carbon sources, polymethylmetacrylate (PMMA) and Polydimethylsiloxane (PDMS). The utilization of reactive hydrogen plasmas enable the graphene growth at reduced temperatures as compared to conventional thermal chemical vapor deposition processes. The effects of substrate temperature on graphene quality were studied based on Raman analysis, and a reduction of defects at elevated temperature was observed. Moreover, one facile approach to incorporate nitrogen into graphene by plasma treatment in a nitrogen/hydrogen gas mixture during the synthesis process was demonstrated. First principle calculations revealed that the incorporated nitrogen atoms with the Pyridinic-N and Pyrrolic-N formats in the graphene sheets would tune the band gap of graphene effectively which may provide a new step toward controlled graphene electronics, and this technique may lead to precise determination of graphene-based device characteristics in the future.
8:00 PM - P8.27
Atomic Dopants Involved in the Structural and Electrical Evolution of Graphene and Its Application for FET Device and Energy Storage Materials
Currently, there is increasing interest in studying the properties of an electron donor atom doped-reduced graphene oxide (rGO) for potential application in electron rich field effect transistor (FET) (called n-type FET) devices and energy storage materials. Thermally doped nitrogen and phosphorous atoms on the sp2-carbon network of rGO enhance its structural and electrical properties because of its ability to donate an electron and form a de-localized structure due to the lone pair electrons of donor atoms.1-3
In this work, we report two kinds of applications using the electron donor atom doped-rGO. Firstly, we report new fabrication method and characterization of n-type graphene through thermal graphitization with nitrogen and phosphorous sources for n-type FETs. Especially, phosphorous doped graphene exhibits superior chemical and electronic stability on the FET device even under oxygen atmosphere environment.1 Furthermore, we like to report an identification on the changes of atomic structure of nitrogen doped graphene by STM. Atomic structural information of thermally annealed rGO provides an understanding on how the hetero-atomic doping could affect electronic property of rGO.4 Finally, we like to report the fabrication method and characterization of highly nitrogen doped porous rGO to use electrode materials for supercapacitor. This material with high surface area (~900 m2/g) seems to show high doping percentage (~20 %) in XPS.
1. S. Some, J. Kim, K. Lee, A. Kulkarni, Y. Yoon, SM. Lee, T. Kim and H. Lee, Adv. Mater. 24, 5481 (2012)
2. S. Some, P. Bhunia, EH. Hwang, K. Lee, Y. Yoon, S. Seo and H. Lee, Chem. Eur. J. 18, 7665 (2012)
3. P. Bhunia, EH. Hwang, Y. Yoon, E. Lee, S. Seo and H. Lee, Chem. Eur. J. 18, 12207 (2012)
4. Y. Yoon, S. Seo, G. Kim and H. Lee, Chem. Eur. J.18, 13446 (2012)
8:00 PM - P8.28
Molecular Orientation Controlling Charge Transport of Graphene Field Effect Transistor
Graphene nanoscaffolds have shown excellent performance in catalysis, chemical sensing, and biomolecule sensing. Understanding the molecular adsorption mechanism and the molecular orientation effect on the graphene surface are very necessary for its application. Here, we report charge transport behaviors of a graphene channel in terms of the nature of two different graphene frameworks and the molecular orientation of the adsorbed functional molecules. Chemically synthesized n-doped graphene (e.g., nitrogen-doped reduced graphene oxide, n-RGO, labeled n-type prone graphene) and chemically vapor deposited graphene (CVD graphene, p-CVD-G, labeled p-type prone graphene) are used as different graphene channels to interact with the adsorbed functional molecules. The functionalized benzene molecules and their diazonium salts are physically and chemically attached to the graphene channel surfaces, respectively. Interaction results of the adsorbed molecules at both graphene channel surfaces are measured by current-voltage transport curves of their field effect transistors. The physically (horizontally) bound molecules on both p- and n-type prone graphene channels produced p- and n-type doping effects depending on the functional group. Alternatively, the chemically (vertically) bound diazonium molecules did not produce molecular doping effects on changes of major charge carriers, but followed the nature of each p- and n-type prone graphene channel. Our novel findings promise a new molecular doping strategy of graphene for its applications.
1. Bekyarova, E.; Itkis, M. E.; Ramesh, P.; Berger, C.; Sprinkle, M.; de Heer, W. A.; Haddon, R. C. J. Am. Chem. Soc. 2009, 131, 1336-1337.
8:00 PM - P8.29
Functionalization of Chemically Reduced Graphene Oxide for Gas Sensor Application
Sensor-based detection of gases is an important task to improve the safety and quality of human life. Highly sensitive metal oxide based gas sensors are well established and widely implemented. However, they operate at high temperatures (250 - 600°C), which requires excessive energy and degrades their long-term stability. In this regard, graphene based gas sensors, which operate at low temperatures (25 - 85°C), could provide a solution. Upon gas adsorption the conductance changes quickly and with high sensitivity. Additionally these materials are scalable and easy to produce.
In this study, graphene was prepared by reduction of graphene oxide (GO), which was obtained by oxidation of graphite (Hummers method). The resulting reduced graphene oxide (rGO) is stable in water, enabling easy transfer to any substrate by spin coating or drop casting. To test the capability of rGO as a gas sensor material, the conductivity of rGO modified electrodes was studied in the presence of various gases (NO2, CO2, CH4, H2 and CO). Upon adsorption the change in resistivity of rGO coated electrodes was very sensitive, especially towards NO2 and CO2. Selectivity was developed by chemical modification. Electrochemical doping with Ni nanoparticles created selectivity towards CO. An increased sensitivity towards H2 was accomplished by chemical doping with Pd and Pt nanoparticles. Doping with TiO2 resulted in selective detection of NO2.
Graphene based sensors provide a low-cost alternative for the detection of hazardous or explosive gases. The detection of CO2 is particularly needed in many practical applications, e.g., for more energy efficient operation of air conditioning.
8:00 PM - P8.30
Doping of CVD Graphene Films Using Amine Containing Polymers
High electrical conductivity and optical transparency make graphene a potential material for use as a flexible transparent conductive electrode in organic electronic devices. For integration into these devices, the electronic energy level between the conductive graphene and the other active layers must be controlled. Here, an ultrathin layer of a polymer containing simple aliphatic amine groups, polyethylenimine ethoxylated (PEIE), was deposited on graphene to lower its work function. Scanning Kelvin probe microscope (SKPM) techniques applied on graphene before and after PEIE deposition and demonstrated that the work function can be lowered over 1 eV. Electrical measurements performed on back-gated field effect graphene devices indicate that shifts in the Dirac point correlates well to the shift in the Kelvin Probe measurements, verifying the changes in graphene doping. Unlike destructive doping techniques of graphene such as substitutional doping or covalent functionalization, this method is defect-free as there is no discernible D peak in Raman spectra of graphene films after PEIE deposition. This method can be easily processed in air, from dilute solutions in environmentally friendly solvents such as water or methoxyethanol and does not suffer any change until a temperature of 190°C, making it compatible with the processing of printed electronic.
8:00 PM - P8.32
Electrical Transport of Zig-zag and Folded Graphene Nanoribbons
In recent years, there has been much interest in modelling graphene nanoribbons as they have
great potential for use in molecular electronics. We have employed the NEGF formalism to determine the conductivity of graphene nanoribbons in various configurations. The electronic structure calculations were performed withiin the framework of the Extended Huckel Approximation. Zig-zag graphene nanoribbons were considered with folded nanoribbon contacts and with silicon contacts to establish the importance of the contact geometry and chemistry on the conductance. The conduction pathways and their dependence on the geometry of the nanowires were also examined. It was found that the geometry of the contacts plays a crucial role in determining the electron transfer pathways. The results of our calculations are compared with that obtained from recent work carried out using tight-binding model Hamiltonians [1,2,3].
. H Takahi and N Kobayashi, Physica E 43 (2011) 711.
 J.W. Gonzalex et al, Solid State Comm 152 (2012) 1400.
 M.Ashhadi and S.A.Ketabi, Phsica E (2012) in press.
8:00 PM - P8.33
Non-conventional Techniques for the Modification of Carbon Nanoforms
Even though the solubility of Carbon Nanoforms in common solvents is really very low, the main methods for transforming them have so far involved the use of traditional chemical techniques, such as refluxing and sonication, in the presence of large amounts of strong inorganic acids or organic solvents, over long periods of time. The modification of carbon nanostructures in the absence of any solvent has many advantages over solution reactions: a) this methodology does not suffer the solubility problems; and b) the solid-state reactions have many benefits: reduced pollution, low costs, and simplicity in process and handling. These factors are especially important for scalable production.
There is an increasing number of publications demonstrating the advantages that technologies such as microwaves and ball milling, can offer for the modification of different compounds, in solvent-free conditions. In the present work we summarize our achievements in the modification of Carbon Nanotubes (CNTs) and Carbon Nanohorns (CNHs). It has been shown that these structures display strong microwave absorbing properties and this behaviour have been used for purification and functionalization. Another stimulating possibility is the ball milling approach where chemical bonds are activated by the presence of an external mechanical force. Both methods pave the way for green protocols and large-scale modifications while avoiding significant degradation of the structure. Finally, ball milling processes have also been applied to the exfoliation of graphite in order to form stable dispersions of graphene sheets.
8:00 PM - P8.34
Correlation Between Extrinsic Defect and Doping in Graphene
All the spectacular properties of graphene are due to the confinement of electrons/holes in a plane of atomic thickness. This makes graphene based devices extremely sensitive to how it is grown on different substrates and how it is doped to obtain n-type and p-type conductivity in graphene layers. Though doping is extremely important for graphene based devices, but doping also introduces defects. Particularly this is extremely important for graphene, so it is desirable to achieve p-type and n-type conductivity in graphene without destroying monolayer induced properties such that high mobility due to linear dispersion relation can be retained. Here, we report the correlation between external defects incorporated due to supramolecular functionalization (doping) of graphene sheets. We have intercalated graphitic layers of HOPG by means of sonication to exfoliate graphene layers using different organic molecules, which also act as dopant. We have used toluene, acetone, propylene carbonate and DMF for this purpose. Further, graphene field effect transistors (g-FET) were fabricated by putting Au source-drain electrodes using electron beam lithography. SiO2 (300 nm)/Si(n++) was used as substrate. By analyzing the transfer characteristics (IDS-VDS), large shift in Dirac point (-20V to -40V) was observed in case of graphene exfoliated in acetone, propylene carbonate and DMF, whereas graphene exfoliated in toluene exhibits very small deviation in Dirac point (+0.3V). Position of the Dirac point clearly indicates that graphene layers exfoliated in acetone, DMF and propylene carbonate are p-type and in toluene, it exhibits pristine (undoped) behavior. We have observed a peak known as D band peak at 1350 cm-1 in Raman spectra in case of graphene exfoliated in acetone, propylene carbonate and DMF. This peak is absent in Raman spectra of graphene exfoliated in toluene. This allows us to correlate the extrinsic defects which could arise due to functionalization. DMF, propylene carbonate and acetone have oxygen atom, which attracts electron cloud from graphene sheet during sonication resulting supramolecular functionalization which can be correlated with D band in Raman spectra and the large shift in Dirac point, whereas toluene has no oxygen and also less polar compared to acetone, propylene carbonate and DMF. So, by choosing particular organic solvent for intercalation, it is possible to control doping and doping induced defects in graphene. Functionalization of graphene sheets was also analysed by monitoring shift in respective Raman peak positions, UV-Vis absorption spectroscopy and IR spectroscopy. Mobility value upto 10000 cm2/V-s was found for pristine graphene and with p-type doping it was reduced to 6000 cm2/V-s due to doping induced defects.
8:00 PM - P8.35
Interfacial Charge Carrier Dynamics in CdSe/Reduced Graphene Oxide Nanoheterostuctures
Due to the excellent electrical conductivity, reduced graphene oxide (RGO) may attract photoexcited electrons of semiconductor nanocrystals to improve the overall charge separation property. Such capability of electron trapping for RGO has rendered it promising potential in relevant photoconversion processes such as photocatalytic water splitting, photocatalysis and photovoltaics [1-3]. Here we presented the fabrication of CdSe/RGO nanoheterostructures and investigated the interfacial charge carrier dynamics using time-resolved fluorescence spectroscopy. The samples were prepared by depositing CdSe nanocrystals on the surface of RGO in the hydrothermal process. Note that RGO was obtained from typical Hummers’ method. By applying the multiple chemical exfoliation treatment , the lateral size of RGO can be decreased to 50 nm and 5 nm. With the reduction of the characteristic size of RGO and thus the regulation of its work function, a further improved charge carrier separation for CdSe/RGO can be achieved. Time-resolved fluorescence spectra were measured to quantitatively analyze the electron transfer event between CdSe and RGO for CdSe/RGO and its dependence on modulation of the relative band structures. The correlation of charge carrier dynamics of CdSe/RGO with the result of their performance evaluation in photocatalysis may provide insightful information when using semiconductor/graphene nanoheterostructures in photoelectrochemical system for photoconversion applications.
 X. Y. Zhang, H. P. Li, X. L. Cui, and Y. Linb, J. Mater. Chem. 2010, 20, 2801-2806.
 Q. P. Luo, X.Y. Yu, B. X. Lei, H. Y. Chen, D. B. Kuang, and C.Y. Su, J. Phys. Chem. C. 2012, 116, 8111−8117.
 S. Sun, L. Gao, Y. Liu, and J. Sun, Appl. Phys. Lett. 2011, 98, 093112.
 B. D. Pan, J. Zhang, Z. Li, and M. Wu, Adv. Mater. 2010, 22, 734-738.
8:00 PM - P8.36
Controllable Doping of n-Type Graphene Transistors with Extended Air Stability
This work presents an innovative approach to fabricating controllable n-type doping graphene transistors with excellent air-stability by using self-encapsulated doping layers of titanium sub-oxide (TiOx) thin films, which are an amorphous phase of crystalline TiO2 and can be solution processed. The non-stoichiometry TiOx thin films consisting of a large number of oxygen vacancies is used as “active” n-type doping agent. A novel device structure consisting of both top and bottom coverage of TiOx thin layers on a graphene transistor exhibited strong n-type transport characteristics with its Dirac point shifted up to -80 V and an enhanced electron mobility with doping. The n-type doping density could be controlled with different concentrations of TiOx, and it can also be further tuned by illuminating ultra-violet light, which makes it possible to precisely control n-type doping level of graphene transistors. In addition to work as a controllable n-doping material, the TiOx film could simultaneously act as an effective encapsulated layer. An extended stability of the device without rapid degradation after doping was observed when it was exposed to ambient air for several days, which is not usually observed in other n-type doping methods in graphene. The technique of using an “active” encapsulated layer with controllable and substantial electron-doping on graphene provides a new route to modulate electronic transport behavior of graphene and has considerable potential for the future development of air-stable and large area graphene-based nano-electronics.
 “Self-encapsulated doping of n-type graphene transistors with extended air stability”, ACS Nano, Vol. 6, 6251, 2012
8:00 PM - P8.37
The Effect of Synthesis Temperature on the Properties of Nitrogen-doped Carbon Nanotubes
Tailoring the properties of carbon nanotubes (CNTs) to obtain materials suitable for various applications is an important subject that has attracted the attention of many researchers. One of the most effective ways to tune the physical and chemical properties of carbon nanotubes is to dope them with a foreign element, such as nitrogen. Nitrogen atoms that are incorporated into the CNTs structure can drastically change the properties of CNTs. The main effects are to change the number of defects in the structure and to alter molecular orbitals.
The properties of nitrogen-doped carbon nanotubes (N-CNTs) originate from its structure, thus it is practical to create a desired structure during synthesis. Here we have investigated the effect of the synthesis parameters, mainly temperature, on various characterizations of N-CNTs such as their morphologies, dimensions (diameter and length), defects, nitrogen inclusions and thermal stability. The N-CNTs were synthesized via Chemical Vapor Deposition (CVD) method using ammonia as the source of nitrogen and ethane as the source of carbon.
The results revealed strong correlations between the synthesis parameters and properties of synthesized N-CNTs. TEM images showed bamboo-like morphology at all temperatures, indicating that the growth mechanism was not altered by synthesis temperatures between 550°C and 950°C. As well, electron microscopy images revealed the presence of longer N-CNTs at higher synthesis temperatures. The diameter of nanotubes was also found to increase with increasing synthesis temperatures. XPS characterization indicated that the percentage of nitrogen inclusion decreases with increasing synthesis temperature. A TGA study demonstrated a trend of increasing thermal stability of N-CNTs with increasing synthesis temperature, up to 850°C. Based on these results by choosing the appropriate synthesis conditions, one can tune N-CNTs to control properties and obtain materials with the desired characteristics.
8:00 PM - P8.39
In Situ Electronic Characterization of Graphene Nanoconstrictions Fabricated in a Transmission Electron Microscope
Merchant, Zhengqing John
We report electronic measurements on high quality graphene nanoconstrictions (GNCs) fabricated in a transmission electron microscope (TEM), and the first measurements on GNC conductance with an accurate measurement of constriction width down to 1 nm. To create the GNCs, freely suspended graphene ribbons were fabricated using few-layer graphene grown by chemical vapor deposition. The ribbons were loaded into the TEM, and a current-annealing procedure was used to clean the material and improve its electronic characteristics. The TEM beam was then used to sculpt GNCs to a series of desired widths in the range 1-700 nm; after each sculpting step, the sample was imaged by TEM and its electronic properties were measured in situ. GNC conductance was found to be remarkably high, comparable to that of exfoliated graphene samples of similar size. The GNC conductance varied with width approximately as G(w) = (e2/h)w^0.75, where w is the constriction width in nanometers. GNCs support current densities greater than 120 μA/nm2, 2 orders of magnitude higher than that which has been previously reported for graphene nanoribbons and 2000 times higher than that reported for copper. This ranks GNC as a potential candidate for the interconnect application in the future electronics. (This work was supported by the National Science Foundation through Grant #DMR-0805136 and NIH Grant R21HG 004767 and R21HG 557611.Use of facilities of the Nano/Bio Interface Center at UPenn is gratefully acknowledged.)
8:00 PM - P8.40
Sub-10-nm Block Copolymer Patterns for the Graphene Nano-ribbon Array Field Effect Transistors
Graphene has great interests because of its incredible electronic properties for the potential future electronic devices, such as high electrical mobility, high thermal conductivity and their transparency, from molecular-levelly thin two-dimensional materials. However, their intrinsical zero bandgap property is fairly hard to perform the high on-off current ratio of field effect transistor. For the overcoming this huddle, many researchers have studied opening bandgap of graphene with versatile methods. Among these attempts, graphene nano-ribbon with sub-10 nm structures for the quantum confinement was suggested and showed opening bandgap and high on-off ratio experimentally. However, to achieve a high driving current for the practical device application, densely aligned graphene nano ribbon array should be required.
A block copolymer is well-known material for the straightforwardly developing nano-sized pattern from sub-10 nm to 100 nm on large area through the self-assembly. Thus, this self-assembly based patterning materials has been intensively studied for the smaller sized pattern applied to semiconducting and hard disk industry. Our group particularly has investigated highly aligned sub-10 nm pattern formation from polystyrene-block-polydimethylsiloxane (PS-b-PDMS) block copolymer, which is representative for the high block copolymers, and their complex patterns to the application to practical devices.
In this presentation, we demonstrate sub-10-nm graphene nano-ribbon array field effect transistors which fabricated by cylindrical PS-b-PDMS block copolymer line patterns as a lithographical template on chemical vapor deposition (CVD) grown graphene monolayer sheets.
8:00 PM - P8.41
Nitrogen Doped Vertical Few-layer Graphene Films: Synthesis and Material Analysis
As the sibling of vertically aligned carbon nanotubes, vertical few-layer graphene network film (VGF), simply based on geometry consideration, can provide superior performance than laterally or randomly oriented graphene sheets based film (or graphene paper) for many applications, particularly for energy storage. Here we report our synthesis and material studies of VGFs, especially nitrogen-doped VGFs, for rapid supercapacitor applications.
VGFs and nitrogen in-situ doping were obtained by microwave plasma chemical vapor deposition. Nanostructure, electrical, and chemical properties of VGFs were studied. In particular, the electrochemical properties of doped VGF were investigated with the aim for energy storage. The supercapacitor performance was evaluated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge measurements. Our results suggest that VGFs are of great potential to realize KHz supercapacitors. Terahertz (THz) time-domain spectroscopy was also used to determine the frequency dependent complex permittivity and conductivity of VGFs from DC up to THz.
8:00 PM - P8.42
Carbon Monoxide-induced Reduction and Healing of Graphene Oxide
van de Sanden, Cristian
Reduction of graphene oxide (GO) has recently generated intense research interest due to the possibility of using this method to inexpensively produce large quantities of graphene. Current reductive processes rely on thermal or chemical removal of oxygen functional groups from the surface. While reduction has been demonstrated, 10-12 atomic percent oxygen remains on the surface of GO after processing with current techniques. Furthermore, use of high process temperatures, ~1000 °C, in the reduction of GO leads to the generation of defects through the loss of carbon atoms in the basal plane. Therefore, the ultimate improvement in the electronic, optical, or mechanical properties of graphene that can be achieved through reduction of GO is limited by the defect formation and residual oxygen that remains after reduction. Here, we report the facile removal of oxygen functional groups from the surface through the reduction of GO in a carbon monoxide atmosphere. Common oxygen containing functional groups on the basal plane (epoxides, hydroxyls, ketone pairs) are removed from the surface of GO with CO2 and H2O being the only gas-phase products. We have used molecular dynamics and density functional theory calculations to elucidate the mechanisms for the reaction of CO with the functional groups on GO: these reactions are facile and proceed without degradation of the graphene sheet. The removal of oxygen functional groups from GO by CO is also confirmed experimentally using in situ attenuated total reflection Fourier transform infrared spectroscopy. Interestingly, we also observe that CO can heal defects in the basal plane of graphene. In a controlled set of experiments, we generated defects in a pristine graphene sheet grown on Cu foils and subsequently exposed it to CO: analysis of the D, G, and 2D Raman bands of graphene shows a near-complete healing of point defects such as monovacancies. Thus, our results indicate that CO-induced reduction of GO proceeds without damaging the underlying basal plane, producing reduced GO with superior properties.
8:00 PM - P8.43
η6 Chemical Modification of Epitaxial Graphene: An Avenue for Non Destructive Surface Functionalization and Atomic Layer Deposition
Graphene’s superior properties are expected to develop next generation electronic applications. However, a major challenge which still remains is its functionalization to incorporate it into different systems and applications without altering its superior properties. Generally, functionalization leads to the conversion of the planar sp2 hybridized state of carbon into the tetrahedral sp3 states. This conversion leads to increase in carrier scattering and reduction in carrier density and deterioration of graphene’s properties. In this talk, we demonstrate an η6 functionalization route, where the d-orbital of a transition metal binds with the pi-cloud of the graphene. The result is preservation of the sp2 state of carbon atoms and the superior properties of graphene (high carrier density and low carrier scattering). Performance of several sensors based on η6 graphene will also be discussed. We envision that this functionalization route will significantly broaden graphene’s scope of applications.
8:00 PM - P8.46
Growth and Characterization of Graphene for Satellite and Rocket Applications
We have been studying graphene applications in satellite and launch vehicle technology. In particular we have examined the chemical vapor deposition of graphene from various hydrocarbon sources such as methane, ethane and propane and their effect on graphene properties. The larger molecules are found to more readily produce bilayer and multilayer graphene, due to a higher carbon concentration and different decomposition processes. However, single- and bilayer graphene can be grown with good selectivity in a simple, single-precursor process by varying the pressure of ethane. We will present data on graphene grown using this method and others along with graphene’s potential applications including chemical sensors and thermal conduction.
8:00 PM - P8.48
Wetting Applications for Graphene: Superhydrophobic Foams and Low Hysteresis Coatings
Controlling the interaction of water with a solid surface is a classical and important problem in surface engineering. For a droplet resting on a solid, the angle formed between the solid-liquid interface and liquid-vapor interface in the side view is defined as the water contact angle. Higher the contact angle, the more water repellant a surface is. A material with a contact angle higher than 90° is termed hydrophobic while that above 150° is superhydrophobic. Low surface-energy materials and air-entrapping roughness features are typically used to achieve superhydrophobicity. High droplet mobility is also quintessential as it enables self-cleaning surfaces and lab-on-chip devices among other applications. When the droplet moves, the advancing front has a higher contact angle than the receding front and their difference is termed as contact angle hysteresis. Low hysteresis is desirable since it indicates lesser pinning of water droplets and hence greater mobility.
Here we explore the wetting behavior of graphene and investigate these properties in freestanding, three-dimensional, superhydrophobic graphene architectures. Template-directed chemical vapor deposition is used to grow micro-porous graphene foam, which has a lot of trapped air pockets that facilitate water droplets to stay suspended over them. In this Cassie wetting state, the droplet stays above the pores and exhibits low contact angle hysteresis. A fluoropolymer coating was used to prevent a Cassie to Wenzel state transition, wherein the droplet displaces air and completely wets the walls of the pores. An advancing angle of 163° and a receding angle of 143° were achieved. Even in rough situations such as droplet impact, the Cassie state is shown to be extremely stable and the droplet effectively rebounds off the surface without pinning to or damaging the structure. Lower hysteresis associated with the Cassie state allows droplets to roll off easily thereby removing contaminants. Such mechanically robust, superhydrophobic foams show potential for a variety of applications in self-cleaning surfaces, anti-contamination and low friction coatings.
Further, the wetting transparency of graphene was also studied and hysteresis significantly reduced on hydrophobic surfaces with nano roughness features. Monolayer to few layer graphene was used to drape the nanostructured surfaces and effect a hysteresis reduction from ~115° to as low as ~20°. This dramatic drop is once again attributed to prevention of a Cassie to Wenzel state transition. The mechanism here depends on the wetting transparency and impermeability of graphene rather than relying on a low surface energy material. The ability to minimize hysteresis on nanostructured materials without using a low surface energy material thus opens up new avenues for scalable and low cost production of lab-on-chip devices and anti-stick coatings.
8:00 PM - P8.49
Reduced Graphene Oxide and Graphene Oxide for High Performance Dye-sensitized Solar Cells
Dye-sensitized solar cells (DSCs) have attracted considerable attention as the third generation solar cells because of their low fabrication cost and promising photon-to-current conversion efficiency (PCE). Two-dimensional reduced graphene oxide (rGO) and graphene oxide (GO) were incorporated into the (1) counter electrodes, (2) electrolyte and (3) photoanode of DSCs to improve their photovoltaic stability and fabrication efficiency. (1) Mesoporous rGO films fabricated by gel coating had been used as the platinum-free counter electrodes of DSCs. The rGO films could effectively catalyzed the electrochemical reduction of the triiodide ions, yielding a PCE of 7.19%, the highest efficiency for iodide/triiodide DSCs with graphene as the counter electrode. DSCs using the rGO counter electrode is more stable than that using the platinum counter electrode. (2) Amphiphilic GO having a hydrophobic core and hydrophilic side groups was used to gel 3-methoxyproponitrile (MPN). GO loading as low as 2.5wt.% was required for the gel formation. The 2.5wt.% GO-MPN organogel incorporated with iodine, 1-methyl-3-propylimidazolium iodide, guanidinium thiocyanate and 4-tert butylpyridine was used as the electrolyte in quasi-solid-state DSCs. DSCs with the GO-MPN organogel electrolyte exhibited a PCE of 6.37%, under AM1.5 illumination. DSCs with the gel electrolyte had better photovoltaic stability than its liquid electrolyte counterpart. (3) The optimal thickness of the mesoporous TiO2 photoanode is approximately 13-15µm. The fabrication of a thick and crack-free TiO2 film requires multiple printing of the TiO2 paste. A new TiO2 paste was formulated by incorporating GO into the paste. The GO served as an auxiliary binder, enabling the fabrication of a 13-15µm thick TiO2 film free of crack via a single print. DSCs fabricated from the GO-incorporated TiO2 paste had a PCE of 7.70%, similar to that fabricated from the conventional TiO2 paste without GO (7.76%). The discovery of this new TiO2 paste would greatly increase the fabrication efficiency of DSCs.
8:00 PM - P8.50
Transparent and Flexible Graphene-platinum Films for Advancing the Performances of Dye-sensitized Solar Cells
Transparent films of Pt nanoparticles on graphene were fabricated by solution casting and annealing process involving a home-made polymeric dispersant. The dispersant consisting of poly(oxyethylene)-segment and cyclic imide functionalities in the structure was prepared and used to stabilize the graphene dispersion in aqueous medium. In the second step, the in situ reduction of added dihydrogen hexachloroplatinate in ethanol afforded the homogenous dispersion of Pt nanoparticles (4.0 nm diameter) on graphene nanohybrids. Subsequently, the solution casting on polyimide substrate and annealing process allows the fabrication of large-scale dimension films that are most suitable for using as the counter electrolyte (CE) of dye-sensitized solar cells (DSSC). The characterization of the films by TEM showed the ultimate formation of Pt metal interconnected network on the surface of the 2D graphene aggregated roughness. The optimization of Pt/graphene at 5/1 w/w had led to the films that demonstrated a high counter-electrode catalytic activity and the DSSC efficiency of 8.00%, significantly outperformed the conventional platinum sputtered CE (7.14%). Moreover, the fabricated CE films appeared to be transparent and flexible for allowing the device of rear-illuminated DSSC. The graphene-Pt CE DSSC had exhibited a relatively high power efficiency of 7.01% in contrast to the merely 2.36% for the DSSC employing the conventional sputtered Pt CE, under the tests of rear-side illumination. The difference relies on the film transparency of the CE layer. Analyses of cyclic voltammetry, incident-photo-to-current efficiency and electrochemical impedance spectra were well correlated to the high efficiency of the cell performance. The use of polymeric dispersants and graphene-Pt nanohybrid is proven to be a viable approach for promoting the performances of DSSC.
8:00 PM - P8.51
Electronic Transport and Molecular Sensing Properties of Large-area Nitrogen-doped Graphene Sheets
Berkdemir, Ana Laura
Large-area (~4 cm2) and highly-crystalline monolayer nitrogen-doped graphene (NG) sheets have been synthesized on copper foils by ambient-pressure chemical vapor deposition (AP-CVD) method. As-grown graphene sheets could be easily transferred from copper foils onto other substrates (e.g. silicon/silicon dioxide wafers, TEM grids). Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal that the nitrogen dopants in as-synthesized NG samples are separated by one carbon atom and sit consequently on the same sub-lattice of graphene. Based on our first principles and tight binding calculations, this unbalanced distribution of dopants on one of the graphene sub-lattices will promote the opening of an electronic band gap. We control the synthesis parameters and use Raman spectroscopy and electrical transport measurements to monitor the nitrogen doping levels. Finally, we will demonstrate that NG behaves as an efficient molecular sensor, especially when performing graphene-enhanced Raman scattering (GERS) of various organic and bio-molecules.
8:00 PM - P8.52
Parts-per-quadrillion Gas Detection with Pristine Carbon Nanotubes and Graphene
The capability to detect ultra-low concentration of various substances vital for human activities is progressing rapidly owing to recent advances in nanomaterials. Carbon nanotubes (CNTs) and Graphene are promising nanomaterials for sensor applications. However, it is challenging to exploit their unique properties, particularly high surface to volume ratio, to achieve optimal sensitivity due to their vulnerability against contaminations. Here we show that despite considerable achievements have been made in the last several years, we are still far from what a pristine graphene or CNT can truly offer. By applying continuous in situ UV light illumination in the course of detection, we have observed 2 to 4 orders of magnitude better sensitivity than current state-of-the-art results for a range of gas molecules, and for the first time entered parts-per-quadrillion (PPQ) detection level at room temperature [1, 2]. We attribute this astonishing performance to UV light induced sensor surface cleaning during the detection. The result points out a route to exploit the intrinsic sensitivities of other nanomaterials.
 G. Chen, T. M. Paronyan, E. M. Pigos, and A. R. Harutyunyan, Scientific Reports 2, 343 (2012).
 G. Chen, T. M. Paronyan, and A. R. Harutyunyan, Appl. Phys. Lett. 101, 053119 (2012)
8:00 PM - P8.53
Few-layer Graphene - PbSe Nanoparticle Hybrids for Photodetectors
Hybrids of graphene with high charge carrier mobilities and semiconductor nanoparticles (NPs) with size-tunable absorption are ideal building blocks for optoelectronic devices. Coupling near-infrared absorbing PbSe NPs to graphene transforms the photoexcited states of the NPs into charge separated states. Holes are transferred to graphene, whereas electrons remain trapped in the PbSe NPs. Due to the fast charge transport in graphene a dramatic increase of photosensitivity is expected. Thus stable graphene - PbSe NP hybrids are promising materials for the fabrication of highly sensitive flexible near-infrared photodetectors.1,2
Here we demonstrate a facile synthesis of few-layer graphene (FLG) - PbSe NP hybrids, their structural and chemical characterization (e.g. by transmission electron microscopy (TEM), Raman spectroscopy, absorption spectroscopy, infrared spectroscopy) and investigation of their response to light.
Growth of PbSe NPs on exfoliated FLG flakes was carried out using the hot-injection method. Adding FLG to a lead precursor solution leads to the coordination of the metallic precursor on the FLG flakes. The injection of a selenium precursor solution results in a direct growth of PbSe NPs on FLG without any molecular linker. Depending on the growth time, the NP size was controlled and the spectral responsivity of the hybrids was tuned. TEM images indicate that PbSe NPs with cubic crystal structure were selectively deposited on the FLG surface. Due to solvent exfoliation, FLG is free from oxidation/reduction induced defects and the high charge carrier mobility of graphene is not disturbed. Current-voltage characteristics of the FLG - PbSe NP hybrids are studied in the dark and under near-infrared illumination.
8:00 PM - P8.54
Charge Transfer and Gate-modulated Positive/Negative Photoconductivity in Graphene CdSe Quantum Dot Hybrid Devices
Graphene is a potential material for optoelectronics and photodetection owing to its high carrier mobility and transparence. However, its application is limited by the lack of spectral selectivity and its low photo-absorption of ~2.3%. We demonstrate a hybrid device that composes of monolayer graphene decorated with a layer of colloidal cadmium selenide quantum dots (CdSe QDs), exhibiting positive and negative photoconductivity upon irradiation with wavelength close to the quantum dots’ excitonic absorption. The sign of the photoconductivity can be electrically controlled by modulating the charge carrier in graphene via back-gate voltage. We show that such positive/negative photoconductivity can be related to the charge transfer between QDs and graphene in the hybrid devices and we have studied the dependence of such charge transfer on QDs deposition, thermal activation and light illumination.
8:00 PM - P8.56
2D and 3D Graphene-based Chemical Sensors on Flexible Substrates
Graphene has been investigated for the applications in solar cells, transistors, sensors and light-emitting diodes due to its unique properties such as high mechanical, electrical, optical and thermal properties. 2D graphene film is one of the ideal materials for chemical sensors because of its high surface to volume ratio and facile integration with other electronic devices. 3D graphene foam can enhance the conductivity and mechanical durability further while maintaining the superior properties of the 2D graphene film. Also porous structure of 3D graphene foam is an extra advantage in chemical sensors. In this study, we demonstrated graphene-based NO2 chemical sensors by using 2D graphene film and 3D graphene foam on both flexible PET and paper substrates.
Chemical vapor deposition grown graphene film and foam were transferred to PET and clean room paper substrates, respectively. In the case of PET substrates, Au electrodes were sputtered with shadow mask before the transfer of graphene layer. In the case of paper substrates, graphene film and foam were turned over to expose graphene with the PMMA layer on back, followed by transfer to the paper substrate. Finally, two electrodes were formed by silver paste. Micro-Raman spectroscopy was conducted to characterize quality and thickness of 2D- and 3D graphene structures. Electrical behaviors under NO2 and air ambient were investigated under various strain conditions by monitoring I-V characteristics and time-dependent dynamic response. Our flexible graphene-based sensors on both PET and paper substrates showed rapid response under various strain conditions after the introduction of 200 ppm of NO2¬ gas due to the charge transfer between graphene and NO2 molecules, which act as p-type dopants in graphene. Highly sensitive graphene sensors on flexible substrates were demonstrated on PET and paper substrates. Compared with monolayer graphene, the 3D graphene foam showed higher conductance and better mechanical stability. The details of our experiments and results will be presented.
8:00 PM - P8.57
Modification of Electronic Properties of Top-gated Graphene Devices by Ultrathin Yttrium-Oxide Dielectric Layers
We report our systematical studies on the structures and electrical properties of single layer graphene (SLG) top-gated devices using Y2O3 as dielectric layers. The ultrathin (~5nm) Y2O3 layers fabricated for the sandwiched device of SiO2/graphene/ Y2O3 are structurally uniform and dense, which is verified by the characterizations of AFM, TEM and Raman spectroscopy. Moreover, these SLG field-effect transistors (FETs) at cryogenic temperatures have excellent performance with transconductance up to 13000 μF/Vs (gmN=μCox), which is among the highest in graphene FETs reported previously. The modification of electronic properties (due to insignificant underlying disorder) by Y2O3 layers on top of graphene have also been well studied including transport and quantum capacitance measurements. Based on Boltzmann transport theory concerning the change of dielectric environment, Coulomb scattering is confirmed quantitatively to be dominant in the sandwiched device and very few short-range impurities have been introduced by Y2O3. Both DC transport and AC capacitance measurements at high magnetic fields demonstrate that Landau levels have broadened more obviously than uncovered graphene on SiO2, which is caused by the additional charge impurities and enhanced inhomogeneity of carriers introduced by Y2O3 layers.
8:00 PM - P8.58
Tuning the Work Function of Graphene with a High Molecular Weight Electron Acceptor
Graphene is an emergent candidate as transparent electrode material for next generation (opto-) electronic devices. Grown by chemical vapour deposition on copper foil results in large-area polycrystalline graphene sheets with excellent structural and electronic properties that can be transferred to virtually any other substrate . The use of graphene as a transparent electrode in organic electronic devices, such as light emitting diodes and solar cells, requires matching its work function to the hole and electron transport levels of the organic semiconductors, in order to minimize charge injection barriers. This can, in principle, be achieved by depositing strong electron acceptor or donor molecules that undergo a charge transfer on the surface and thus induce surface dipoles. However, many potential electron acceptors, such as tetrafluoro-tetracyanoquinodimethane (F4TCNQ), are too volatile for practical applications. In this work, we show with photoelectron spectroscopy that the work function of graphene on glass can be continuously increased from 3.7 eV up to 5.5 eV with (sub-) monolayer coverage of the strong organic molecular acceptor hexaazatriphenylene-hexacarbonitrile (HATCN), which is practically relevant due to its high molecular weight (384 g/mol). A charge transfer type interaction between HATCN and graphene is verified by a low binding energy emission in the N 1s core level region and the observation of a presumably filled level derived from the lowest unoccupied level of HATCN. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy indicates a coverage-dependent re-orientation from flat-lying to vertically inclined HATCN in the monolayer regime, in analogy to what was reported for HATCN deposited on silver (Ag) surfaces . This structure change may explain the sub-linear increase of the work function as function of HATCN coverage.
 Li, X. S. et al.; Science 324, 1312-1314 (2009)
 Bröker B.et al.; Phys. Rev. Lett., 104, 246805 (2010)
8:00 PM - P8.59
A Simple Hydrazine-assisted Method for the Growth of Monodisperse SnO2 Nanoparticles on Graphene for Lithium Ion Batteries
Lithium ion batteries (LIBs) have now become important power sources because of their high energy density and high voltage. Commercial anode materials in LIBs are usually graphitic due to their intrinsic stability and low cost. However, because graphitic materials have a relatively small specific capacity (372 mAh g-1), these materials cannot the performing demanding task of high capacity storage. Therefore, alternative anode materials with higher specific capacity such as metals, metal oxides, and metal sulfides should be substituted for the graphitic materials.
Among the metal oxides, SnO2 is an attractive material for the fabrication of negative electrodes for lithium ion batteries because of its high theoretical reversible capacity of 782 mA h g-1, which is more than twice that of graphite. Unfortunately, insertion of lithium ion induces a large volume change, which causes rapid degradation of the cycling performance and even cracking of electrode.
Graphene, an atomic single layer of honeycomb carbon lattice, has attracted great attention because of its superior electronic conductivity, high surface area (theoretical value 2600 m2 g-1), and physicochemical stability. Graphene used as substrate for metal oxide nanoparticles already showed to improve the mechanical stability and the electrochemical performances. Therefore, graphene-metal oxide composites are good candidate as anode for lithium ion batteries. However, generally, graphene-metal oxide composites are prepared in several steps before a final annealing aimed to improve the crystallinity of the oxide and to reduce graphene oxide. Therefore, known processes are commonly complicated and time-consuming.
In this study, we introduce a facile one-pot process for the fabrication of SnO2 nanoparticles-graphene nanocomposites based on the hydrothermal synthesis assisted by hydrazine, which control the SnO2 nanoparticles formation and crystallization as well as the reduction of graphene oxide to graphene. This strategy provides a highly crystalline oxide and more complete reduction of graphene oxide. The SnO2-graphene composite consists of 3-4 nm monodisperse SnO2 nanocrystals uniformly dispersed at the surface of graphene. When used as anode material for lithium ion batteries, it exhibits a first discharge capacity of 1662 mA h g-1, which rapidly stabilizes and still remains at 626 mA h g-1 even after 50 cycles, when cycled at a current density of 100 mA g-1. Even at the very high current density of 3200 mA g-1, the composite displays stable capacity of 383 mA h g-1 after 50 cycles. It is evident that the graphene framework has greatly improved the electron transfer and the stability of electrode compared to pure SnO2. The results prove that graphene-based heterostructures synthesized by this novel hydrothermal method are good candidates for anode materials in lithium ion batteries. Finally, this synthesis approach can be probably extended and generalized to large variety of metal oxides.